The present invention relates to a method for fabricating a pressure sensor entirely from silicon. More particularly, a method is disclosed for fabricating a pressure sensor incorporating Silicon-On-Insulator ("SOI") material, and offering a high degree of immunity to temperature effects, increased reliability, minimal substrate parasitics, and reduced manufacturing variations from device to device. Additionally, the sensor architecture and fabrication method disclosed allow for the production of an inexpensive, simple and reliable device.
The prior art contains many different kinds of monolithic silicon absolute pressure sensors; virtually all of them measure the pressure required to deflect a thin diaphragm, usually made of silicon or silicon dioxide, which forms one wall of a vacuum-sealed reference cavity. Generally, the diaphragm deflection results in either a change in the capacitance between electrodes mounted on the diaphragm and the base of the sensor (capacitance type sensor), or a stress build-up at the edge of the diaphragm which is sensed by a thin film or implanted resistor (piezoresistive type sensor). The electrical output of the devices is directly related to changes in pressure upon the diaphragm.
To date, the use of these types of sensors in high-performance applications has been limited due to a number of problems associated with their manufacture and operation. In addition, these sensors are typically very sensitive to temperature effects (especially the piezoresistive type sensors where the temperature dependence of the silicon material itself affects the electrical output). Temperature variations also have adverse effects upon the capacitive type sensors, as most of them employ metal film electrodes within their reference cavities. Differences in thermal expansion coefficients between the diaphragm (usually composed of silicon or silicon dioxide) and the metal film electrodes can result in changes in electrode separation as the temperature changes, causing false or misleading outputs. In severe cases, this effect could lead to the delamination of the metal film electrode from the diaphragm, or to the cracking of the diaphragm itself. Moreover, since the electrodes are inside of the reference cavity, metal leads to the outside must pass through a vacuum seal. These leads are prone to failure at these vacuum seal points when exposed to repeated temperature cycling.
The accuracy of these sensors has also been limited by the inability to hold tight tolerances on the diaphragm thickness during batch fabrication of these devices. Any variation in diaphragm thickness from device to device would result in a change in the amount of deflection which results from a given pressure differential, and therefore result in erroneous pressure readings. While this non-uniformity can be compensated for by individually calibrating each sensor, such a task would be time consuming and costly.
Accordingly, it is the object of the present invention to provide for a method of fabricating an all-silicon absolute pressure sensor employing SOI technology, which allows for improved operational temperature stability, reduced substrate parasitics, increased immunity to thermally induced mechanical fatigue, and tighter diaphragm thickness tolerance control from device to device. This sensor is based upon an ungated Metal-Oxide Semiconductor Field-Effect Transistor ("MOSFET"), the drain current (also referred to as "channel current") of which varies as a function of the deflection of a diaphragm located in close proximity to the device.